Noncontact type ic card

ABSTRACT

The present invention relates to an electronic device that suppresses reduction in reception sensitivity and occurrence of distortion when IC cards are used while stacked. A loop antenna  1  equipped to the IC card has four turns, and it is designed so that one turn comprises a linear portion L 2,  a curved line portion L 3,  a linear portion L 4  and a curved line portion L 5,  for example. When two loop antennas  1  having such a shape is stacked with being faced in the opposite directions, the corners thereof are not overlapped. For example, the corner formed of the curved line portion L 3  and the linear portion L 4  of the loop antenna  1  is overlapped with the curved line portion L 3  of the loop antenna  1  superimposed with being faced in the opposite direction, so that the corners are not overlapped and the overlap portion is reduced as a whole. The present invention may be applied to an IC card for receiving/transmitting data under non-contact state.

TECHNICAL FIELD

[0001] The present invention relates to an electronic device, andparticularly to an electronic device for suppressing reduction of areception efficiency at maximum and performing the processing on thebasis of signals having little distortion when data are written into orread from plural IC cards under a non-contact state while the plural ICcards are stacked.

BACKGROUND OF THE INVENTION

[0002] Recently, non-contact type IC (Integrated Circuit) cards havebeen popular, and they have been introduced as common tickets totransportation facilities such as subways, buses, or ferries and forelectronic money systems, etc. The data writing/reading into/from the ICcards are carried out in the non-contact style on the basis of theprinciple of electromagnetic induction.

[0003] That is, in the non-contact IC card system, electromagnetic waveis emitted from a loop antenna having a reader/writer forwriting/reading data into/from an IC card, and the electromagnetic waveemitted is received by a loop antenna equipped in the IC card, wherebycommunications are made between the IC card and the reader/writer.

[0004] Further, in order to eliminate a load imposed on an IC cardmaintenance work, the IC card is provided with no battery, and thus itis designed to achieve required power from the electromagnetic wavereceived. Therefore, the IC card is required to receive electromagneticwave as efficiently as possible.

[0005] In general, the IC card is equipped therein with a loop antenna 1having two or more turns of wire formed in a rectangular shape which isas large as possible as shown in FIG. 1. The loop antenna 1 receivessignals by resonating a resonance circuit with the carrier frequency ofelectromagnetic wave emitted from the reader/writer.

[0006] The IC card is basically used alone, however, there is a casewhere two IC cards are used together with being stacked. For example,when a user passes over a commuter pass zone and then gets off at astation out of the commuter pass zone, the user presents his/hercommuter pass and an iO card (trademark) (it is assumed that each of thecards comprises an IC card) to a reader/writer while these cards arestacked in order to adjust the surcharge of the over-zone. At this time,the reader/writer or a non-contact IC card system connected to thereader/writer recognizes the zone of the commuter pass thus presented,and then carries out the processing of calculating the extra distancethe user has ridden, calculating the surcharge based on the extradistance of the over-zone and adjusting the account on the iO card.

[0007] Not only when one IC card is presented, but also when plural ICcards are stacked, it is necessary that electromagnetic wave can beefficiently received so that each of the IC cards can operate desiredcommunication operations.

[0008] For example, when two IC cards each having a loop antenna 1having the construction shown in FIG. 1 are stacked with the IC cardplaced face up while being perfectly overlapped with each other in theview from the upper side (when the two IC cards are stacked so that thetop side of one card faces the back side of the other card) as shown inFIG. 2, the linear portions L1 to L9 constituting the respective sidesof rectangles of the conductor constituting the loop antenna 1-1 of oneIC card are overlapped with the corresponding linear portions L11 to L19of the loop antenna 1-2 of the other IC card, respectively. At thistime, the resonance frequency of the resonance circuit of the loopantenna 1-1, 1-2 is reduced to the inverse of the square root of 2(1/{square root}2) of the resonance frequency when only one loop antenna1 is provided (only one IC card is provided).

[0009] That is, when the IC cards are overlapped, coupling occursbetween the resonance circuits thereof. Therefore, the resonancefrequency is equal to 1/(2π{square root}(LC)) in the case of one ICcard, however, it is equal to 1/(2π{square root}(L·(2C))=(1/{squareroot}2)·1/(2π{square root}(LC)) in the case of two stacked IC cards.That is, when IC cards are stacked, the resonance frequency of theresonance circuit of each IC card is displaced from the carrierfrequency of the electromagnetic wave by 1/{square root}2, and thereception efficiency is reduced.

[0010] However, for example when the carrier frequency of theelectromagnetic wave emitted from the reader/writer is equal to 13.56MHz, by setting the resonance frequency of the resonance circuit of eachIC card to 17.5 MHz higher than the carrier frequency in advance, theelectromagnetic wave can be received not only when only one IC card isused, but also even when two IC cards are used while they are stacked soas to be faced in the same direction.

[0011] When IC cards are stacked while one IC card is placed face up andthe other IC card is placed face down as shown in FIG. 3 (two IC cardsare stacked while both the top sides thereof or both the back sidesthereof face each other), both the linear portion L1 of the loop antenna1-1 and the linear portion L13 of the loop antenna 1-2, both the linearportion L2 of the loop antenna 1-1 and the linear portion L12 of theloop antenna 1-2 and both the linear portion L3 of the loop antenna 1-1and the linear portion L11 of the loop antenna 1-2 are respectivelybasically overlapped with each other, however, the overlapping style ismore complicated than that of FIG. 2.

[0012] The theoretical ground has not been necessarily clear, however,it has been discovered from at least experiment results that theresonance frequency is further reduced as compared with the case whereIC cards are stacked as shown in FIG. 2. As described above, the methodof setting the resonance frequency of the resonance circuit to a valuehigher than the carrier frequency in advance in consideration of thecase where plural IC cards are stacked has such a problem that it canachieve a sufficient effect both when one IC card is used and when twoIC cards are stacked so as to be faced in the same direction as shown inFIG. 2, however, it cannot achieve a sufficient effect when two IC cardsare stacked so as to be faced in the opposite directions as shown inFIG. 3.

[0013] Further, there is a problem that when two IC cards are stacked,reception signals contain distortion.

DISCLOSURE OF THE INVENTION

[0014] The present invention has been implemented in view of theforegoing situation, and has an object to prevent reduction in receptionefficiency and thus reduce distortion of signals received by designingloop antennas so that it is difficult to couple the loop antennas evenwhen IC cards are overlapped with one another in any way.

[0015] A loop antenna of an electronic device according to the presentinvention is characterized in that the loop antenna is shaped so thateven when plural electronic devices are stacked so that at least partsof the top sides thereof face one another or at least parts of the backsides thereof face one another, the conductors constituting the portionsof the loop antennas which are located in the neighborhood of thecorners of the electronic devices each having a substantiallyrectangular shape are prevented from being overlapped with one another.

[0016] The loop antenna may be designed to have a parallelogram shape orelliptical shape as a basic shape.

[0017] The basic shape of the loop antenna may be constructed as asubstantially annular shape by a combination of two straight lines andtwo curved lines while the two straight lines are disposed in parallelto each other.

[0018] The basic shape of the loop antenna may be constructed as asubstantially annular shape by a combination of two or four curved linesthat are different in shape.

[0019] A loop antenna of an electronic device according to the presentinvention is characterized in that one of two corner portions that aredisposed so as to sandwich the center line therebetween is disposed tobe close to the center line, and the other corner portion is disposed tobe far away from the center portion.

[0020] A loop antenna of an electronic device according to the presentinvention is characterized in that the loop antenna is shaped so thatthe line connecting two points of the loop antenna that are located atthe closest positions to the corners of the electronic device so as toconfront each other is coincident with a diagonal line of the electronicdevice having a substantially rectangular shape.

[0021] An information writing/reading device according to the presentinvention is characterized by including reading/writing means forwriting/reading information into/from plural electronic devices when theplural electronic devices are partially overlapped with one another, theplural electronic devices having loop antennas shaped so that when theplural electronic devices are stacked so that at least parts of the topsides thereof face one another or at least parts of the back sidesthereof face one another, the conductors constituting the portions ofthe loop antennas that are located in the neighborhood of the corners ofthe electronic devices each having a substantially rectangular shape arenot overlapped with one another.

[0022] An information writing/reading method according to the presentinvention is characterized by including a writing/reading step ofwriting/reading information into/from plural electronic devices when theplural electronic devices are partially overlapped with one another, theplural electronic devices having loop antennas shaped so that when theplural electronic devices are stacked so that at least parts of the topsides thereof face one another or at least parts of the back sidesthereof face one another, the conductors constituting the portions ofthe loop antennas that are located in the neighborhood of the corners ofthe electronic devices each having a substantially rectangular shape arenot overlapped with one another.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagram showing a loop antenna 1;

[0024]FIG. 2 is a diagram showing the overlap between loop antennas 1;

[0025]FIG. 3 is a diagram showing the overlap between the loop antennas1;

[0026]FIG. 4 is a block diagram showing an example of the constructionof a non-contact IC card system to which the present invention isapplied;

[0027]FIG. 5 is a circuit diagram showing an example of the constructionof IC 13 of FIG. 4;

[0028]FIG. 6 is a diagram showing the shape of the loop antenna 1;

[0029]FIG. 7 is a diagram showing a signal transmitted from areader/writer 21;

[0030]FIG. 8 is a diagram showing a signal processed by an IC card 11;

[0031]FIG. 9 is a diagram showing a equivalent circuit when two loopantennas 1 are overlapped with each other;

[0032]FIG. 10 is a diagram showing another shape of the loop antenna 1;

[0033]FIG. 11 is a diagram showing a state that two loop antennas 1having the shape shown in FIG. 10 are overlapped with each other;

[0034]FIG. 12 is a diagram showing a signal to be processed by the ICcard 11;

[0035]FIG. 13A is a diagram showing another shape of the loop antenna 1;

[0036]FIG. 13B is a diagram showing another shape of the loop antenna 1;

[0037]FIG. 13C is a diagram showing another shape of the loop antenna 1;

[0038]FIG. 13D is a diagram showing another shape of the loop antenna 1;

[0039]FIG. 13E is a diagram showing another shape of the loop antenna 1;and

[0040]FIG. 13F is a diagram showing another shape of the loop antenna 1.

BEST MODES FOR IMPLEMENTING THE INVENTION

[0041]FIG. 4 shows an example of the construction of an non-contact typeIC card. The IC card 11 is a battery-less type IC card having no batteryfor power supply. A loop antenna 1 constituting a resonance circuit incombination with a capacitor 12 receives electromagnetic wave (indicatedby a solid line) emitted from a reader/writer 21, converts theelectromagnetic wave received to an electrical signal and then suppliesthe electrical signal to IC 13. The inner construction of the IC 13 willbe described later with reference to FIG. 5.

[0042] A host computer 31 controls DPU (Digital Processing Unit) 41 ofthe reader/writer 21 to write predetermined data into the IC card 11 orread out predetermined data from the IC card 11. Further, the hostcomputer 31 processes the data which is input from DPU 41 and read outfrom the IC card 11, and displays the data on a display portion(notshown) as occasion demands.

[0043] DPU 41 generates control signals for various kinds of control onthe basis of an instruction from the host computer 31 and controls amodulation/demodulation circuit 42. In addition, it generatestransmission data corresponding to the instruction and supplies the datato the modulation/demodulation circuit 42. Further, DPU 41 generatesreproduction data on the basis of response data from themodulation/demodulation circuit 42 and outputs the data to the hostcomputer 31.

[0044] The modulation/demodulation circuit 42 modulates the transmissiondata input from DPU 41 and supplies the data to a loop antenna 43. Themodulation/demodulation circuit 42 also demodulates modulated wave fromthe loop antenna 43, and inputs the demodulated data to DPU 41. The loopantenna 43 emits the electromagnetic wave corresponding to the modulatedsignal input from the modulation/demodulation circuit 42.

[0045] Next, the specific construction of the reader/writer 21 and IC 13will be described with reference to FIG. 5. In this figure, themodulation/demodulation circuit 42 of the reader/writer 21 shown in FIG.4 is illustrated as an oscillator 42A serving as a modulation circuitand a demodulation circuit 42B.

[0046] IC 13 which is connected to a resonance circuit comprising theloop antenna 1 and the capacitor 12 of the IC card 11 has a rectifyingcircuit 61. The rectifying circuit 61 comprises a diode 71, a resistor72 and a capacitor 73. The rectifying circuit 61 rectifies and smoothensthe signal supplied from the loop antenna 1, and supplies apositive-level voltage to a regulator 64. The regulator 64 stabilizesthe positive-level voltage thus input, converts it to a DC voltagehaving a predetermined level and then supplies it as a power source to asequencer 66 and other circuits.

[0047] A modulation circuit 62 is connected at the rear state of therectifying circuit 61. The modulation circuit 62 comprises a seriescircuit of an impedance element 81 and FET (Field Effect Transistor) 82,and it is connected to the loop antenna 1 constituting a coil of theresonance circuit in parallel. FET 82 is turns on or off in conformitywith the signal from the sequencer 66 to set a state that the impedanceelement 81 is inserted in parallel to the loop antenna 1 or a state thatit is not inserted in parallel to the loop antenna 1, whereby theimpedance (the load of the loop antenna 43) of the circuitelectromagnetically-coupled to the loop antenna 43 of the reader/writer21 through the loop antenna 1 is varied.

[0048] The signal which has been rectified and smoothened by therectifying circuit 61 is supplied to a high pass-filter (hereinafterreferred to as HPF (High pass-Filter)) 63 comprising a capacitor 91 anda resistor 92, and high-band components thereof are extracted andsupplied to a demodulation circuit 65. The demodulation circuit 65demodulates the signal of the high frequency components input theretoand outputs the signal to the sequencer 66.

[0049] The sequencer 66 has ROM (Read Only Memory) and RAM (RandomAccess Memory) (both are not shown) therein, stores a signal (command)input from the demodulation circuit 65 into RAM, analyzes the signalaccording to a program contained in ROM, and reads out data stored inthe memory 67 on the basis of the analysis result as occasion demands.The sequencer 66 further generates a response signal to return aresponse corresponding to the command, and supplies it to the modulationcircuit 62.

[0050] Next, the operation of the reader/writer 21 and the IC card 11shown in FIG. 5 will be described by taking as examples a case whereinformation is written from the reader/writer 21 into the IC card 11 anda case where information is read out from the IC card 11. The hostcomputer 31 controls DPU 41 of the reader/writer 21, and instructs thewriting of prescribed data into the IC card 11. On the basis of theinstruction from the host computer 31, DPU 41 generates a command signalfor the writing, generates transmission data (writing data)corresponding to the instruction and supplies the data to the oscillator42A. The oscillator 42A modulates the oscillation signal on the basis ofthe signal input, and supplies it to the loop antenna 43. The loopantenna 43 emits the electromagnetic wave corresponding to themodulation signal input.

[0051] The resonance frequency of the resonance circuit comprising theloop antenna 1 and the capacitor 12 of the IC card 11 is set to thevalue corresponding to the oscillation frequency (carrier frequency) ofthe oscillator 42A. Accordingly, the resonance circuit receives theemitted electromagnetic wave by its resonance operation, converts theelectromagnetic wave received to an electrical signal and then suppliesit to the IC 13. The electrical signal thus converted is input to therectifying circuit 61 equipped to the IC 13. The diode 71 of therectifying circuit 61 rectifies the signal input, and the capacitor 73smoothens the signal, and thereby supplying the positive-level voltagethereof to the regulator 64. The regulator 64 stabilizes thepositive-level voltage input, converts it to the DC voltage havingprescribed level, and then supplies the DC voltage as a power source tothe sequencer 66 and other circuits.

[0052] The signal which has been rectified and smoothened by therectifying circuit 61 is supplied through the modulation circuit 62 toHPF 63 to extract high-band components, and the high-band componentsthus extracted are supplied to the demodulation circuit 65. Thedemodulation circuit 65 demodulates the signal of the high-frequencycomponents thus input, and outputs the signal to the sequencer 66. Thesequencer 66 stores the signal (command) input from the demodulationcircuit 65 into RAM, analyzes the signal according to the programcontained in ROM, and writes the writing data supplied from thedemodulation circuit 65 into a memory 67 on the basis of the analysisresult.

[0053] When the command supplied from the demodulation circuit 65 is areading command, the sequencer 66 reads out the data corresponding tothe command from the memory 67. The sequencer 66 turns on or off FET 82in conformity with the data thus read out. When FET 82 is turned on, theimpedance element 81 is connected to the loop antenna 1 in parallel, andwhen FET 82 is turned off, the parallel connection is released. As aresult, the impedance of the load circuit of the loop antenna 43 whichis electromagnetically connected through the loop antenna 1 varies inaccordance with the read-out data.

[0054] The terminal voltage of the loop antenna 43 varies in accordancewith the variation of the impedance of the load thereof. Thedemodulation circuit 42B reads this variation to demodulate the read-outdata and outputs the demodulated data to DPU 41. DPU 41 properlyprocesses the data input thereto, and outputs the data thus processed tothe host computer 31. The host computer 31 processes the read-out datainput from DPU 41, and displays the data on the display portion (notshown) as occasion demands.

[0055]FIG. 6 shows the construction of the loop antenna 1 installed inthe IC card 11 shown in FIGS. 4 and 5. The portions corresponding to theprior art are represented by the same reference numerals, and thedescription thereof is suitably omitted. The loop antenna 1 shown inFIG. 6 has a rectangular shape as a basic shape, and has four turns. Thevalue of the inner angle θ of each of all the four corners is set to90°−α in any turn. That is, the inner angle between the linear portionL1 and the linear portion L2, the inner angle between the linear portionL2 and the linear portion L3, the inner angle between the linear portionL3 and the linear portion L4 and the inner angle between the linearportion L4 and the linear portion L5 are set to 90°−α. Here, the valueof the constant α may be set to any positive or negative value otherthan 0. Accordingly, the respective linear portions are kept innon-parallel state at each side of the rectangle. For example, at theupper side of the rectangle, the linear portions L2, L6, L10 and L14 arekept to be non-parallel to one another.

[0056] The terminals 14-A, 14-B are connected to IC 13. By designing theloop antenna 1 in such a shape, the linear portions of the loop antenna1 are hardly coupled to one another (when viewed from the upper side,the overlap portion of the linear portions is reduced) and thus thereduction in reception efficiency can be suppressed in both the casewhere the two IC cards 11 are stacked with being faced in the samedirection as shown in FIG. 2 and the case where thy are stacked withbeing faced in the opposite directions as shown in FIG. 3.

[0057] As a result, in any case where one IC card 11 is used, two ICcards are stacked with being faced in the same direction or two IC cardsare stacked with being faced in the opposite directions, the receptionsensitivity enough for communications can be achieved.

[0058] Next, FIG. 8 shows the measurement result of a reception voltagereceived by an IC card 11 to which the loop antenna 1 as shown in FIG. 6is applied when a signal having a waveform as shown in FIG. 7 istransmitted from the reader/writer 21 to the IC card 11. In FIG. 7, theaxis of ordinate represents a transmission voltage (V), and the axis ofabscissa represents a lapse time (second). In FIG. 8, the axis ofordinate represents a reception voltage (V), and the axis of abscissarepresents a lapse time (second).

[0059] The voltage V1 of FIG. 7 and the voltage V2 of FIG. 8 havedifferent values, and the axes of ordinate of FIGS. 7 and 8 aredifferent in scale. The measurement result shown in FIG. 8 is ameasurement result when two IC cards 11 each having a loop antenna 1 asshown in FIG. 6 are stacked with being faced in the opposite directionsas shown in FIG. 3.

[0060] Even when a signal having a good waveform as shown in FIG. 7 istransmitted from the reader/writer 21 to the IC card 11, it is processedas a signal having a distorted waveform as shown in FIG. 8 at the ICcard 11. In order to discover the cause for the distortion, a schematicequivalent circuit to the IC card 11 as shown in FIG. 9 is considered.

[0061] In this case, it may be considered that two IC cards 11 are keptto be stacked and the IC cards 11 having the same construction areconnected to each other through a capacitor 101-1 and a capacitor 101-2.The inner construction of the IC 13-1, 13-2 is similar to the innerconstruction of the IC 13 shown in FIG. 5, and it comprises therectifying circuit 61, the modulation circuit 62, etc.

[0062] The reason why the signal to be processed at the IC card 11becomes a signal having distortion as shown in FIG. 8 when the signalhaving the waveform shown in FIG. 7 is transmitted from thereader/writer 21 to the IC card 11 is associated with capacitancerepresented by the capacitors 101-1, 101-2 which are induced by theoverlap between the two loop antenna 1-1 (IC card 1-1) and the loopantenna 1-2 (IC card 1-2). The distortion can be suppressed by reducingthe capacitance.

[0063] The capacitance represented by the capacitors 101-1, 101-2 isdependent on the physically overlapped portion between the loop antenna1-1 and the loop antenna 1-2. That is, as the overlap portion betweenthe loop antenna 1-1 and the loop antenna 1-2 is increased, thecapacitance represented by the capacitor 101-1, 101-2 is increased.Particularly in the loop antenna 1 having a shape as shown in FIG. 6,the lines at the portions other than the linear portions L1 to L16, thatis, at the four corner portions are in closer formation than at thelinear portions. If the loop antennas are prevented from beingoverlapped with each other at the four corner portions, the capacitancerepresented by the capacitors 101-1, 102-2 could be reduced.

[0064] Therefore, a loop antenna 1 having a shape as shown in FIG. 10 isconsidered. In the loop antenna 1 shown in FIG. 10, the portionscorresponding to the loop antenna 1 shown in FIG. 6 are represented bythe same reference numerals. In the shape of the loop antenna 1 shown inFIG. 10, two sides out of four sides comprise a substantially arcuatecurved line L1, L5, L9, L13, L17 and a substantially arcuate curved lineL3, L7, L11, L15 at the confronting position, and the remaining twosides comprise a linear portion L2, L6, L10, L14 and a linear portionL4, L8, L12, L16 at the confronting position.

[0065] In FIG. 10, IC 13 is connected between the terminals 14-A and14-B (not shown), and the capacitor 12 is connected to IC 13 inparallel.

[0066] By setting the two corners out of the four corners to right-angleshape and setting the other two corners to curved-line shape, as shownin FIG. 11 the two loop antennas 1 are prevented from being overlappedwith each other at the corner portions at which the lines are closer toone another even when the two loop antennas 1 are overlapped with eachother with being faced in the opposite directions, and thus the signalreceived can be processed as a signal having less distortion.

[0067]FIG. 12 is a diagram showing a signal to be processed in the ICcard 11 when a signal having a waveform as shown in FIG. 7 is receivedfrom the reader/writer 21 by a substantially one IC card 11 achieved bystacking two IC cards 11 equipped with the loop antenna 1 having theshape shown in FIG. 10 so that the IC cards 11 are faced in the oppositedirections (that is, the IC card 11 including two loop antennas 1overlapped with each other as shown in FIG. 11).

[0068] Comparing the waveform shown in FIG. 8 and the waveform shown inFIG. 12, it is apparent that the waveform shown in FIG. 12 has lessdistortion than the waveform shown in FIG. 8. When an IC card 11 isfabricated so that two loop antennas 1 are overlapped with each otherwith being faced in the opposite directions, the signal received can beprocessed as a signal having less distortion by designing the loopantennas so that the overlap portion thereof is reduced, particularlythe four corner portions are not overlapped. The shape in which the fourcorner portions are not overlapped results in reduction in the overlapportion as a whole.

[0069] As the shape of the loop antenna 1 based on the considerationthat the four corner portions (portions at which the line density ishigh) are not overlapped may be used such a shape as shown in FIG. 13 inaddition to the shape shown in FIG. 10.

[0070] The shape shown in FIG. 13A is a parallelogram, and the upper andlower sides in the figure are disposed in parallel to the sides of theIC card 11.

[0071] The shape shown in FIG. 13B is set so that the two sides out ofthe four sides of the parallelogram shown in FIG. 13A are formed ofcurved lines and the sides of the upper and lower linear portions in thefigure are disposed in parallel to the sides of the IC card 11. Theshape shown in FIG. 13C is set so that all the four sides of theparallelogram shown in FIG. 13A are formed of curved lines, and onediagonal line of this shape is disposed to be coincident with onediagonal line of the IC card 11.

[0072] The shape shown in FIG. 13D is set so that two sides of the foursides of the figure are formed of curved lines and the other two sidesare formed of straight lines, and the curved lines are disposed to beclose to the corners of the IC card 11. The shape shown in FIG. 13E isformed of curved lines of two sides, and the line (diagonal line)connecting the positions at which the curved lines are inverted(corresponding to the corners) is disposed to be coincident with onediagonal line of the IC card 11. Further, the shape shown in FIG. 13F isan elliptical shape, and the major axis of the elliptical shape isdisposed to be coincident with the diagonal line of the IC card 11.

[0073] The shape of the loop antenna 1 shown in FIG. 10 or FIG. 13 isset so that when two IC cards 11 are stacked with being faced in theopposite directions as described above, the corner portions (containingthe corner represented by a curved line) of the loop antenna 1 are notoverlapped.

[0074] In other words, the shape of the loop antenna 1 is set so thatone of corner portions which confront each other with respect to thecenter line of the IC card 11 (for example, the corner portion formed bythe curved line L3 and the linear portion L4 in FIG. 10) is far awayfrom the center line and the other corner portion (for example, thecorner portion formed by the linear portion L4 and the curved lineportion L5 in FIG. 10) is closer to the center line.

[0075] Further, in other words, the shape of the loop antenna 1 is setso that the diagonal line of the IC card 11 is coincident with the lineconnecting two points of the loop antenna 1, one point being located atthe closest position to the corner portion of the IC card 11 and theother point being located at the confronting position (for example, inFIG. 10, the intersecting point between the curved line portion L3 andthe linear portion L4 and the intersecting point between the curved lineportion L5 and the linear portion L6).

[0076] By using the loop antenna 1 having any shape shown in FIG. 10 orFIG. 13 as the antenna of an IC card 11, the reduction in receptionsensitivity can be suppressed even when two IC cards 11 are used whilestacked, and the same reception sensitivity as used when only one ICcard is used can be achieved. Further, the distortion of the signal tobe received and processed can be suppressed. Further, even when pluralIC cards 11 of two or more are used while stacked, the above-describedconcept is effective, and the reduction in reception efficiency can besuppressed at maximum and the distortion can be suppressed even when theIC cards are stacked in any way.

[0077] Accordingly, by using the present invention, such ananticollision system that IC cards 11 are used while stacked can beeasily fabricated.

[0078] In the foregoing description, the loop antenna 1 has four or sixturns. However, the number of turns may be set to any value. Further,the loop antenna 1 is equipped to the IC card 11. However, the presentinvention may be applied to a card-like (planar) electronic device otherthan the IC card.

[0079] The term “system” in this specification means an overallapparatus comprising plural devices, means or the like.

[0080] Industrial Applicability

[0081] As described above, according to the electronic device of thepresent invention, when plural electronic devices are stacked so that atleast parts of the top sides thereof face each other or at least partsof the back sides thereof face each other, conductors constituting theloop antenna portions located in the neighborhood of the corners of theelectronic device having a substantially rectangular shape are notoverlapped with each other. Therefore, the reduction in receptionsensitivity can be suppressed, and the distortion of the signal receivedcan be suppressed.

1. A substantially-planar electronic device having a substantiallyrectangular shape containing a loop antenna, characterized in that saidloop antenna is shaped so that even when plural electronic devices arestacked so that at least parts of the top sides thereof face one anotheror at least parts of the back sides thereof face one another, conductorsconstituting the portions of said loop antennas which are located in theneighborhood of the corners of the electronic devices each having thesubstantially rectangular shape are prevented from being overlapped withone another.
 2. The electronic device as claimed in claim 1, whereinsaid loop antenna is shaped to have a parallelogram shape or ellipticalshape as a basic shape.
 3. The electronic device as claimed in claim 1,wherein the basic shape of said loop antenna is constructed as asubstantially annular shape by a combination of two straight lines andtwo curved lines while the two straight lines are disposed in parallelto each other.
 4. The electronic device as claimed in claim 1, whereinthe basic shape of said loop antenna is constructed as a substantiallyannular shape by a combination of two or four curved lines that aredifferent in shape.
 5. A substantially-planar electronic device having asubstantially rectangular shape containing a loop antenna, characterizedin that one of two corner portions that are disposed so as to sandwichthe center line therebetween is disposed to be close to the center line,and the other corner portion is disposed to be far away from the centerportion.
 6. A substantially-planar electronic device having asubstantially rectangular shape containing a loop antenna, characterizedin that said loop antenna is shaped so that the line connecting twopoints of said loop antenna that are located at the closest positions tothe corners of said electronic device so as to confront each other iscoincident with a diagonal line of said electronic device having asubstantially rectangular shape.
 7. An information writing/readingdevice for writing/reading information into/from a substantially-planarelectronic device having a substantially rectangular shape containing aloop antenna, characterized by including reading/writing means forwriting/reading information into/from plural electronic devices whensaid plural electronic devices are partially overlapped with oneanother, said plural electronic devices having loop antennas shaped sothat when said plural electronic devices are stacked so that at leastparts of the top sides thereof face one another or at least parts of theback sides thereof face one another, conductors constituting theportions of said loop antennas that are located in the neighborhood ofthe corners of said electronic devices each having a substantiallyrectangular shape are not overlapped with one another.
 8. An informationwriting/reading method for writing/reading information into/from asubstantially-planar electronic device having a substantiallyrectangular shape containing a loop antenna, characterized by includinga writing/reading step of writing/reading information into/from pluralelectronic devices when the plural electronic devices are partiallyoverlapped with one another, the plural electronic devices having loopantennas shaped so that when the plural electronic devices are stackedso that at least parts of the top sides thereof face one another or atleast parts of the back sides thereof face one another, the conductorsconstituting the portions of the loop antennas that are located in theneighborhood of the corners of the electronic devices each having asubstantially rectangular shape are not overlapped with one another.